Add fieldtrial testing config for EmojiContextMenu.

Bug: 827404
Change-Id: I628949a3e1f8aac41ac7c76f90cf7ba0108179b5
Reviewed-on: https://chromium-review.googlesource.com/1052491Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Ramya Nagarajan <ramyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557333}

testing/variations/fieldtrial_testing_config.json

@@ -1160,6 +1160,21 @@
             ]
         }
     ],
+    "EmojiContextMenu": [
+        {
+            "platforms": [
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "EnableEmojiContextMenu"
+                    ]
+                }
+            ]
+        }
+    ],
     "EnableAppContainer": [
         {
             "platforms": [